The first episode of "Doctor Who" aired 48 years ago today. "An Unearthly Child" introduced William Hartnell as the First Doctor, and Susan Foreman and William Russell as Barbara and Ian, his first companions.

We're celebrating TARDIS Day with every "Doctor Who" theme from 1963 to the one we know and love today. If you have any of the old Hartnell episodes lying around, this is the perfect excuse to break them out and give them a watch. Happy TARDIS Day!